1. Field of the Invention
This invention relates to an air mixed type spray apparatus for spraying liquid such as paint or the like, and more particularly to a spray apparatus that can selectively produce spray in a conical or fan-shaped pattern and yield a preferable result despite the fact that liquid and air are sprayed under a considerably low pressure.
2. Description of the Prior Art
Basic types of spray applied by a conventional spray apparatus for painting purposes chiefly fall into three types: namely, an air spray type, an airless spray type and an air mixed spray type.
Generally, in an air spray type apparatus, paint is emitted from a circular paint spraying hole under a considerably low pressure, and a vast amount of air is jetted from an annular air spraying hole formed around the paint spraying hole under a relatively high pressure. The stream of this jetted air causes paint to be atomized. The air spray type apparatus is adapted to painting to which metallic paint is applied. However, if necessary, various types of auxiliary air holes are formed around the annular air spraying hole to promote the atomization of paint or regulate a spray pattern.
In an airless spray type apparatus, paint which is pressurized to a considerably high pressure in excess of several tens kg/cm.sup.2 or thereabouts is sprayed from an orifice type paint spraying hole, and the paint is atomized by colliding with ambient air, thereby generally producing a fan-shaped spray.
An air mixed spray has been developed in which advantages of both the air spray and the airless spray are incorporated, and advances have been made in putting this spray into practical use (for example, see Japanese Unexamined Patent Publications No. 59(1984)-139958 and No. 62(1987)-160156, and Japanese Utility Model Publication No. 4(1992)-48823). The feature of the air mixed spray is that paint is sprayed from an orifice type paint spraying hole under a relatively high pressure of several kg/cm.sup.2 to several tens kg/cm.sup.2 or thereabouts and a fan-shaped spray is formed. Air for promoting the atomization of paint which has a lower pressure as compared with the pressure of the paint is added to this spray from outside. The air for promoting atomization is added to the spray in such a way as to surround it. This prevents the scattering of the spray and leads to an improved efficiency of adhesion of coating. Various auxiliary air streams are added to a spray in order to improve it and correct the shape thereof. In addition to the prior techniques as set forth, there are, for example, Japanese Unexamined Patent Publications No. 58(1983)-109153 and No. 4(1992)-18952.
Heretofore, the suppression of destruction of the environment, the improvement of the working environment, the safety management of work, and the saving of resources and energy, or the like, have been advocated. Recently, these have become particularly strictly required in painting work. The painting work now encounters the important subjects: an improvement in efficiency of adhesion of paint by preventing the scattering of a spray; the suppression of the amount of volatile diluting solvent used in paint; and the suppression of a jetting pressure as well as a reduction in pressure loading of paint and air to as low as possible simultaneously with the suppression of the amount of pressurized air used in painting.
In practice, in the prior art as is shown in the Unexamined Patent Publications and the Unexamined Utility Model Publication set forth above as the examples of conventional techniques, it is important to set the jetting pressure of paint to substantially 20 kg/cm.sup.2 or thereabouts and set the jetting pressure of air to more than 5 kg/cm.sup.2 or thereabouts. In order to reduce the degree of pressure, it becomes necessary to set the viscosity of paint to a considerably low level by increasing the amount of diluting fluid. Thus, it is quite difficult to reduce the jetting pressure while the viscosity of the paint is maintained or increased.
A drop in discharging pressure, particularly, the discharging pressure of air, affects the environment in front of a spray apparatus. Specifically, the conditions of the addition of air to spray; namely, a collision angle of air to spray, a collision strength, and the symmetrical property of air streams with each other are easily abnormally affected. This results in a significant imperfection, that is, a spray having an uneven particle diameter and a disordered distribution pattern. This also requires a paint nozzle and an air nozzle which are manufactured with high accuracy, thereby resulting in a considerably expensive manufacturing cost and hence rendering the spray apparatus disadvantageous.
Moreover, in the airless spray apparatus which uses an orifice type spray nozzle, it is difficult to obtain a practically circular spray. Particularly, in one type of spray apparatus that uses a fan-shaped spray nozzle with a lip-like discharge orifice, once a fan-shaped spray is formed, and air streams are blown to the edges of that fan-shaped spray to make the spray circular inevitably, the diameter of the spray is reduced, and a circular pattern having a practical diameter is not obtained.
The apparatus disclosed in aforementioned Japanese Unexamined Patent Publication 58(1983)-109153 produces an effect when air is sprayed under a high pressure. Two pairs of nozzles that are disposed opposite to each other must be axially aligned precisely to each other, and also the positional relationship between the nozzles and nozzle chips thereof must be strictly maintained. If these are not achieved, the shape of a fan-shaped spray pattern is disturbed, or initial objects, i.e. an even fan-shaped spray pattern and an improved spraying state cannot be attained, thereby rendering this apparatus disadvantageous. Moreover, this technique basically belongs to an airless spray type, and the chief object thereof is to prevent so-called tailing from occurring in a fan-shaped spray produced by an airless type spray. For this reason, relatively highly pressurized air is locally supplied to a fan-shaped spray from a nozzle having a small diameter. Also, the airless type spray apparatus produces a spray at a high pressure, and therefore this high pressure brings about poor color tone or uneven metallic color when this apparatus is applied to metallic painting. Thus, this apparatus cannot be applied to metallic painting.
One example of a spray gun that realizes a reduced amount of air and a reduced air pressure is disclosed, for instance, in Japanese Unexamined Patent Publication 4(1992)-18952. In this spray gun, air is caused to collide with both edges of fan-shaped spray jetted from a paint nozzle from outside, so that the fan-shaped spray is temporarily concentrated on the center. Thereafter, a fan-shaped spray is formed while it is shifted 90.degree. away from the fan-shaped pattern jetted from the paint nozzle. Accordingly, it is difficult for the spray gun to obtain a fan-shaped spray having a wide angle. To obtain a fan-shaped spray having a wide angle, it is necessary to supply strong air for flattening purpose. This makes it difficult to obtain an orderly arranged fan-shaped spray, and the efficiency of adhesion of paint drops because of the considerable scattering of spray. A conical spray has a considerably narrow angle, and therefore a practical conical spray cannot be obtained.